Wellbores are typically completed with a cemented casing across the formation of interest to assure borehole integrity and allow selective injection into and/or production of fluids from specific intervals within the formation. It is necessary to perforate this casing across the interval(s) of interest to permit the ingress or egress of fluids. Several methods are applied to perforate the casing, including mechanical cutting, hydro-jetting, bullet guns and shaped charges. The preferred solution in most cases is shaped charge perforation because a large number of holes can be created simultaneously, at relatively low cost.
In formations where the sand is porous, permeable and well cemented together, production (i.e., the recovery of hydrocarbons from a subterranean formation) is ideal; that is, it is easier to extract large volumes of hydrocarbons from the formation and into production wells. However, in poorly consolidated formations where the rock material is poorly cemented, sand tends to flow into the wells during production, a problem known as sand production. If the sand reaches the surface, it can damage oilfield hardware and equipment, potentially leading to major failures. In addition, when the solid materials reach the surface, they must be separated from the fluids and disposed of using environmentally approved methods. Moreover, sand production can lead to poor performance in wells and lost production.
To control sand and prevent it from entering a well in order to obtain high production rates from such reservoirs typically requires some means of filtering formation material out of the fluid as it is drawn from the reservoir. Since poorly consolidated formations generally fail under the pressure drawdown applied to them during production, steps must often be taken to control the influx of solids that might otherwise plug or erode and cause the failure of subsurface and surface infrastructure. Once it is determined that a reservoir may be prone to sanding, traditional methods can be implemented to provide a barrier to sand so that it does not enter the well with the hydrocarbons. The methods are typically chosen based on the physical characteristics of the reservoir. For example, sand control measures, such as mechanical filters known as “sand screens” and the packing of gravel around such filters, are often implemented to deal with sand production problems which would otherwise lead to undesirable events such as wellbore collapse and equipment failure. Various sand control techniques have evolved for either limiting the influx of solids, or constructing a mechanical filter to retain loose solids at the sand face, or co-producing solids with the hydrocarbons in a controlled manner.
The most common method of controlling sand production is the installation of one or more sand screens during well completion. Sand screens filter or “screen” the flow of hydrocarbons as they enter the wellbore, allowing fluids to easily pass while preventing sand entry. FIG. 1 illustrates a prior art method for the perforation of sanding prone completions wherein a sand screen 30 is used as a mechanical filter. Screens 30 may be used as filters by sizing the screen to block the flow of particles larger than a given size. Traditionally, a sieve analysis is performed on samples of the formation sand prior to completion of the well and the formation sand particle size range is determined. A filter screen aperture size is chosen which will allow the sand particles to bridge effectively across the screen apertures but not unduly block them. A common criterion for determining screen aperture width is six times the median particle size diameter (6 D50).
The installation of a stand-alone mechanical filter, around which produced solids will accumulate over time to form a natural sand pack filter, is sometimes appropriate. Such installations, however, are vulnerable to erosion of the mechanical filter due to high velocity ingress of fluids through a limited number of inflow points. For example, if a high percentage of perforated tunnels are blocked with debris 22, the fluid inflow from a formation is forced to enter through the few open tunnels, subjecting the filter 32 adjacent to the formation's open tunnels to high erosion because the fluid flow impinges directly onto the filter material at high velocity. A further effect of the influx of formation fluids through a limited set of perforations is an increased risk of sand production due to the high flux rate through the few open tunnels available. The propensity for erosion can be reduced by maximizing the number of perforations open for influx, or by circulating gravel into place around the sand screen to act as a primary filter.
FIG. 2 illustrates a prior art method of completing failure-prone formations to restrain sand production. Gravel packing is accomplished by placing a screen 30 in the wellbore across the intended production zone, then filling the annular area between the screen 30 and the formation 12 with appropriately sized, highly permeable sand 42. The gravel pack sand 42 is sized so that it will not flow into the production equipment but will block the flow of formation sand into the wellbore. Ideally, uniform gravel packing is desired in all tunnels, in order to create an effective filter. However, in reality, ineffective gravel placement often occurs, creating voids 40 within the annular area. This phenomenon is exacerbated by uneven leak-off of fluid from the wellbore into the formation as a result of plugged perforation tunnels. The resulting voids 40 may lead to damage of the filter as a result of erosion 32, also known as “hot spotting”, causing premature failure of the sand filter during production. Big-hole charges, designed to create perforations with a large diameter entrance hole of about 0.8-1.0 inches in diameter are typically used in sand control completions to create as much open flow area (cross sectional area of the holes) in the casing as possible, so as to avoid issues such as hot-spotting and erosion. Perforation tunnel length and geometry is generally less important when using these big-hole charges. While gravel packing has evolved into a complex science, ineffective gravel placement within the perforation tunnels due to the insufficient clean up of perforation tunnels remains a significant problem.
Prior art methods of minimizing sand production without installation of a mechanical filter require that the pressure drop applied across each perforation be minimized to limit rock failure, and the flux rate through each contributing perforation tunnel be minimized to limit the transport of loose grains. This can be achieved by limiting the drawdown applied during production and by maximizing the number of perforations open for influx. However, the latter often requires secondary clean-up activities such as inducing surge flow (at risk of catastrophic sand production) or pumping a clean-up treatment such as an acid to remove soluble debris from blocked perforation tunnels. Creation of surge flow requires running additional equipment and creates a risk of producing undesired amounts of material into the wellbore.
Consequently, there is a need for an improved and economical method for cleaning up tunnels and for substantially sand-free production from failure-prone formations. Such methods should allow for control over or minimization of the production of unwanted sand. The method should adequately clean tunnels without the need for running additional equipment that could cause an influx of sand into the wellbore. The method should eliminate the need for secondary cleanup activities prior to production and/or installation of a sand control completion. Finally, there is a need for a method that provides for the minimization or elimination of any risk of failure of the sand control or production equipment.